develops from external sensory organ precursor cell external sensory organ precursor cell I external sensory organ precursor cell IIa FBbt:00005166 Second-order precursor cell that develops from an external sensory organ precursor cell I. In the microchaete lineage, it divides to give rise to the tormogen cell and the trichogen cell of an external sensory organ (Gho et al., 1999; Hartenstein and Posakony, 1989). In the development of antennal olfactory sensilla, it produces two daughters, pOa and pOb, which each divide asymmetrically to produce the four types of support cell per sensillum (Endo et al., 2012; Chai et al., 2019). pIIa fly_anatomy.ontology SOPIIa